The Chevrolet Volt's gas-electric powertrain is seriously complex. The mechanicals and four primary driving modes -- which we covered earlier this week -- are fairly straightforward, but the Volt continues to hide curiosities in the computer code and control strategies that define this game-changer's behavior. At yesterday's media drive in suburban Detroit, we learned the details of a rare scenario in which the Volt operates with reduced power during extended-range driving.
Reduced power mode exists because of the disparity between the power outputs of the electric motor that drives the wheels and the generator that provides electricity once the gas engine has kicked on. The traction motor is capable of producing 149 hp, while the generator can only turn out 74 hp. Even when the battery has reached "customer empty," it retains 20 percent of its charge, a small sliver of which is used as a buffer when a driver requires more than 74 hp, as they might for a quick acceleration or to climb a large hill. In typical conditions, the generator will refill the buffer range when the driver backs off or the road levels out. But stay on the throttle long enough or find a large enough hill and you'll hit GM's true floor for the battery charge, around 15 percent, that's there to prevent permanent damage to the pack. In this situation, the maximum power is limited to the amount of power that can be transferred directly from the generator to the traction motor. That's 74 hp. To move a 3700-pound car.
Admittedly, only a small percentage of owners will ever experience reduced power in their Volt. The conditions to stress the battery that severely are exceptionally rare in normal driving situations. GM engineers tell us that one real-world scenario for reduced power mode would be a steep grade several miles long, such as you might find in the Rocky Mountains. With just 74 hp available, we're told the Volt would certainly make it up and over the slope, but might do so at, say, 55 mph rather than 70 mph. "You might not be happy at the speed you'd be going," says chief powertrain engineer Pam Fletcher.